(a) Field of the Invention
The present invention relates to a method and apparatus for calibrating a time constant, and a low pass delta sigma modulation apparatus including the same. More particularly, the present invention relates to a method of calibrating values of a resistor and a capacitor having changing characteristics by a process-voltage-temperature (PVT) variation in a low pass delta sigma modulation apparatus.
(b) Description of the Related Art
For high spectral efficiency, a next generation wireless communication system such as 3rd generation partnership project (3GPP) long term evolution (LTE) and worldwide interoperability for microwave access (WiMAX) uses a modulated wideband signal through orthogonal frequency division multiplexing (OFDM). Therefore, development of a high efficiency transmitter for reducing DC power consumption is necessary. Accordingly, an effort for maximizing efficiency has been executed through various structures of power amplifiers such as a Doherty power amplifier and an envelope tracking amplifier. Such a power amplifier is characterized in that a non-constant envelope signal is applied to an input of the power amplifier, and when a signal having a large peak to average power ratio (PAPR) is applied to an input of the amplifier, non-linearity of the amplifier increases and thus there is a limitation in improving efficiency of the power amplifier.
In order to overcome a drawback of such a power amplifier, a structure that can embody a transmitter using a switch mode power amplifier (SMPA) instead of a general power amplifier was suggested.
An input to the SMPA is limited to a signal having a constant envelope, and in order to generate such an input signal, a method such as an envelope delta-sigma modulator (EDSM) and envelop pulse width modulation (EPWM) was suggested. Because the SMPA always operates in a saturation region through such a modulation apparatus, linearity of a non-constant envelope signal is guaranteed and high switching efficiency is obtained. A structure including both an SMPA and a modulation apparatus for converting a non-constant envelope signal to a constant envelope signal is referred to as a class-S power amplifier.
A modulation apparatus using an EDSM method is formed based on a polar modulation method, and the modulation apparatus uses a low pass delta-sigma modulator (LPDSM).
A transmitter in a wireless communication system should satisfy an out-of-band emission specification such as a spectrum mask or an adjacent channel leakage power ratio (ACLR). Quantization noise that is generated in the LPDSM of a modulation apparatus using an EDSM method attenuates such output spectrum characteristics, and thus a design of the LPDSM has an important influence on performance of an entire transmitter. When an LTE input signal having a bandwidth of 20 MHz is applied, in order to obtain error vector magnitude (EVM) characteristics of less than 4%, the LPDSM should have SNR characteristics of about 30 dB or more, and an SNR performance of 40 dB or more is requested in consideration of peak to average power ratio (PAPR) characteristics of 10 dB or more and a design margin.
When designing the LPDSM, one of items to importantly consider is a PVT change amount. Actually, in order to insensitively respond to such a PVT change amount, active elements such as an amplifier or a quantizer and a digital-to-analog converter (DAC) are designed to have a sufficient margin. Therefore, performance of a circuit may not be degraded by some change amount.
However, when passive elements such as a resistor and a capacitor are designed to have a margin like active elements, an RC time constant that is designed to operate in a specific frequency changes. When the RC time constant changes in the LPDSM, quantization noise of the LPDSM is shifted within a bandwidth, and a loop filter becomes unstable and thus the SNR is entirely attenuated. Because of this, a method of designing passive elements to have a margin cannot be used. Therefore, in order to constantly maintain operation of a circuit according to a change amount of the PVT, a method of adding specific calibration is used.
In order to avoid degradation of performance of the LPDSM by maintaining an RC time constant, a resistance that is changed according to a change amount of the PVT should be compensated.
A method of compensating a resistance that is changed according to a change amount of the PVT includes a method of compensating a capacitor value and a feedback current value of a DAC without changing a changed resistance. This method should adjust a current amount of the DAC. It is very high difficult to design a circuit to adjust a current of the DAC, which is one of portions that have a direct influence on noise characteristics and that should be minutely designed in the LPDSM.
Further, a method of compensating a resistance that is changed according to a change amount of the PVT includes a self-calibrating method. The self-calibrating method has a structure that analyzes a noise-transfer function (NTF) of most LPDSMs or in-band noise characteristics, and that feeds back a result thereof and that compensates values of a resistor and a capacitor. This method should use a separate block that analyzes the NTF and use a very complicated algorithm and thus there is much difficulty in embodying an actual analog circuit.